Generally, in air conditioning apparatus, thermal control is accomplished by intermittent operation of the compressor in response to a signal from a thermostat located in the room being cooled. Once the temperature in the room has been lowered to a desired temperature, the refrigerant capacity of the air conditioning system generally need not be very large in order to handle supplemental cooling because of further temperature changes in the room or for keeping the room at the desired temperature. Accordingly, after the room has cooled down to the desired temperature, the most common technique for controlling the output of the compressor is by intermittent operation of the compressor. However, this intermittent operation of the compressor results in the intermittent application of a relatively large load to the driving mechanism of the compressor in order to drive the compressor.
In automobile air conditioning compressors, the compressor is driven by the engine of the automobile through an electromagnetic clutch. Automobile air conditioning compressors face the same intermittent load problems described above once the passenger compartment reaches a desired temperature. Control of the compressor normally is accomplished by intermittent operation of the compressor through the electromagnetic clutch which couples the automobile engine to the compressor. Thus, the relatively large load which is required to drive the compressor is intermittently applied to the automobile engine.
Furthermore, since the compressor of an automobile air conditioner is driven by the engine of the automobile, the rotation frequency of the drive mechanism changes from moment to moment, which causes the refrigerant capacity to change in proportion to the rotation frequency of the engine. Since the capacity of the evaporator and the condenser of the air conditioner does not change, when the compressor is driven at high rotation frequency, the compressor performs useless work. To avoid performing useless work, prior art automobile air conditioning compressors often are controlled by intermittent operation of the magnetic clutch. However, this again results in a large load being intermittently applied to the automobile engine.
One solution to above mentioned problems is to control the capacity of the compressor in response to refrigeration requirements. One construction to adjust the capacity of a compressor, particularly a wobble plate type compressor, is disclosed in the U.S. Pat. No. 3,861,829 issued to Roberts et al. Roberts et al. discloses a wobble plate type compressor which has a cam rotor driving device to drive a plurality of pistons and varies the slant angle of the slant surface to change the stroke length of the pistons. Since the stroke length of pistons within cylinders is directly responsive to the slant angle of the slant surface, the displacement of compressor is easily adjusted by changing the slant angle. Furthermore, change of the slant angle is accomplished by a pressure difference between a suction chamber and a crank chamber in which the driving device is located.
In such a prior art capacity adjusting mechanism, construction of the refrigeration requirements is complicated and enlarges the outer dimension of the compressor. Also, since a large bore is formed through both the wobble plate and cam rotor for penetration of the drive shaft and for enabling the change of the slant angle, the wobble plate causes useless vibration.